Your must haves?

[Fluorescent0]

ElvUI
BigWigs
Skada
Weak Auras
Reflux

[ScooterPerpetual]

WeakAuras and Bigwigs thats it.

[Mikael123]

Bartender (or similar)
Weakauras
If you're tanking: Tidyplates and Threatplates (you can configure the colors to vary by threat status)
If you're healing: Vuhdo

[Deleted]

Like others said before, my UI is also full of must-haves...

But if you just want some good addons to have, i'd say Needtoknow and Tellmewhen. Weakauras may do everything that they do (and better?), but those 2 are much easier to use.

[NMX-]

Bagnon
Bartender4
omnicc
recount
Scrap
(your choice of unit frame)
TidyPlates
WeakAuras

[mitbrandir]

Poweraura. I play a demo lock with UVLS, so i must know when it procs since the proc only lasts 4 seconds. without the addon i would barrely be able to make use of the trinket. Affdot is nice too for other specs, but not so essential as demo where corruption is not something to worry about and doom shouldnt be recast without the 100% ( unless you have no doom and no proc ofc )

[Daws001]

Bartender 4
Grid 2
Clique
Shadowed Unit Frames
Sexy Map
Profiles: Action Bars
Profiles: Keyboard Binds
ClassTimer
TipTac
Quartz
OmniCC
DBM
AtlasLoot
Bagnon

[Halicia]

Genie.

My life was hell when it broke and its a major reason I can't get into other MMOs.

[Cyrops]

npcscan and tomtom

[Siksx]

As a pvp'er I only use OmniCC, Interrupt Bar and Gladius

[Nobleshield]

My go-to list is something like, assuming you don't use a packaged UI like ElvUI or LUI:

* DBM or BigWigs (still torn, I like BigWigs but DBM makes it easier to notice things), for obvious reasons
* Bartender4 (it always seemed better and more customizable than Dominoes)
* Skada (better than Recount in almost every way) or something like TinyDPS if that's all you care about
* Shadowed Unit Frames (pretty customizable with 95% of what you need) or Pitbull if you like to customize everything
* OmniCC
* WeakAuras (BEST. ADDON. EVER.)
* Some kind of chat addon (e.g. Chatter, Prat) because the default chat sucks
* TidyPlates, because default nameplates are garbage

That's about it for my "must have" addons, everything else is QoL stuff (e.g. bag addon, buff addon) or things for specific tasks (e.g. auction addon)

[godded]

Weak Auras
Bartender
Omnicc
Alot
Chatter
Skada (if you care for it)

[IAmBizMarkie]

OmniCC - Action bar cooldown timer. I'm honestly not sure why this should be an add-on at this point.

Grid - Raid frames. Not sure about the modern state of it, but there used to be other Grid-associated add-ons that enhanced the default Grid add-on with mana bars, timers, etc.

Clique - Bind spells to your KBM. Bind Heal to left click, hover over desired unit frame, left click casts heal on said unit.

Bagnon - Makes your character invincible. In no way does this add-on merge your bags into one uniform window.

OmniCC is about my only "Can't Live Without" add-on, but there are really tons of QoL add-ons out there. Just browse WoWInterface and Curse and find what looks good to you.